This is a competing continuation application for a K05 Senior Scientist and Mentorship Award to support Dr. Patricia H. Reggio and her long-standing program in research, education and academic development devoted to understanding the determinants for the action of the cannabinoids at a molecular level. Dr. Reggio's laboratory is currently funded through March 2014 (R01 DA03934) for a multidisciplinary project entitled, Molecular Determinants of Cannabinoid Activity. This project is in its twenty-fourth year of NIDA support. Dr. Reggio currently has a K05 award from NIDA (5K05 DA021358) which ends in April 2012. This award has permitted the reduction of significant teaching and service responsibilities such that she could devote 75% of her time to research over the K05 project period. Under the plan detailed in this K05 application, Dr. Reggio will be able to continue to focus 75% of her time on research, while maintaining her mentorship and leadership activities, as well as her career development. The Research Plan outlines a collaborative, multidisciplinary project that involves the application of state-of-the-art methods for molecular simulation and structure-function analysis, as well as the implementation of new tools in bioinformatics. This plan includes (1) the identification of allosteric modulator binding sites at the cannabinoid CB1 receptor and the synthesis of second generation positive allosteric modulators; (2) the determination of the cannabinoid CB2/ GGDP complex structure based upon novel crosslinking experiments and modeling studies; and (3) the development of novel, selective GPR55 (a putative cannabinoid receptor) antagonists based upon the results of a high-throughput, high-content screen of GPR55 conducted by the Sanford-Burnham screening center of the Molecular Libraries Probe Production Centers Network (MLPCN) in collaboration with Dr. Reggio and Dr. Mary Abood (Temple University). At each step, the computational aspects of this project will be aided and supplemented by collaboration with experimental medicinal chemists, pharmacologists and molecular biologists. Hypotheses generated via molecular simulation will be tested by compound synthesis and pharmacological evaluation. Key receptor residues and interactions identified through receptor modeling studies will be tested via mutation and functional studies. Experimental information will be used to refine receptor models such that at any given time, these models reflect the current state of knowledge in the cannabinoid field. Information about cannabinoid receptor structure and binding modes of ligands will aid in fundamental structure-function studies of this important class of receptors and will also aid in th design of improved therapeutic agents based on the cannabinoids.